System and method for drive-side guarantee of quality of service and for extending the lifetime of storage devices

ABSTRACT

A storage device has a storage medium, a plurality of read-write mechanisms, a quality monitoring and book-keeping unit and a scheduling unit. The plurality of read-write mechanisms is coupled to the storage medium. The quality monitoring and book-keeping unit is coupled to the plurality of read-write mechanisms and is adapted to monitor at least one performance parameter associated with each read-write mechanism during operation. The scheduling unit is coupled to the quality monitoring and book-keeping unit. The scheduling unit is adapted to rank each of the plurality of read-write mechanisms according to the at least one performance parameter and to responsively schedule use of a read-write mechanism according to its rank.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Divisional of and claims priority of U.S.patent application Ser. No. 11/085,845, filed Mar. 22, 2005, the contentof which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to data storage devices, andmore particularly, to storage devices adapted to guarantee a quality ofservice for data transmission and storage.

BACKGROUND OF THE INVENTION

Data storage devices typically store data on a storage medium, whichwill be accessed by a read-write mechanism many times over the life ofthe storage device. As used herein, the term “storage device” refers toany apparatus adapted to store data electronically or magnetically,including disc drives, flash memory, read-only memory (ROM),programmable ROM (PROM), erasable PROM (EPROM), electrically erasablePROM (EEPROM), probe storage devices, and the like.

Over the lifetime of a storage device, a read-write mechanism of thestorage device can experience wear and tear, causing a gradualdeterioration of the performance of the read-write mechanism.Deterioration of the read-write mechanism can include reduced signalamplitude, undesired inter-symbol (or inter-track) interference,excessive noise, unanticipated changes in magnetic field area,misalignment of the read/write mechanism, and numerous other problems,any of which can lead to data errors and/or to failure of the storagedevice. For example, shock events, such as physically dropping thestorage device, can cause the read-write mechanism to contact thestorage medium or can cause misalignment of the read-write mechanism.Additionally, thermal events, such as thermal aspersities, can causewear in the read-write mechanism. As used herein, the term “thermalasperity” refers to a large voltage generated in the read-writemechanism by contact with the storage medium of the storage device.

Deterioration of the read-write mechanism leads to degradation of thesignal quality of an associated read channel, which directly translatesinto inferior quality of service (QoS) and shortened device lifetime ofthe storage device. Quality of service (QoS) refers to a guarantee of aminimum standard of quality for information contained within a signal.Devices that support QoS-guarantees typically provide different levelsof quality depending on which type of data is being processed, such asvoice, data or video. For example, in one instance, a higher quality ofservice may be required for video storage and retrieval (e.g. constantstream of data) than is required for sending and receiving other typesof data (such as sound). In some communication protocols, quality ofservice is maintained using a combination of parity bit checking, errorchecking, encoding and handshaking. Typically, QoS is maintained usingsoftware and/or hardware of a host system. Unfortunately, read mechanismdeterioration can undermine QoS if information received underQoS-guarantees is stored on a storage device using a degraded read-writemechanism, which can introduce errors into the information stream due tophysical deterioration.

There is ongoing need for storage devices that can support QoSguarantees for different types of information. Embodiments of thepresent invention provide solutions to these and other problems, andoffer other advantages over the prior art.

SUMMARY OF THE INVENTION

A storage device has a storage medium, a plurality of read-writemechanisms, a quality monitoring and book-keeping unit and a schedulingunit. The plurality of read-write mechanisms is coupled to the storagemedium. The quality monitoring and book-keeping unit is coupled to theplurality of read-write mechanisms and is adapted to monitor at leastone performance parameter associated with each read-write mechanismduring operation. The scheduling unit is coupled to the qualitymonitoring and book-keeping unit. The scheduling unit is adapted to rankeach of the plurality of read-write mechanisms according to the at leastone performance parameter and to responsively schedule use of aread-write mechanism according to its rank.

In one embodiment, a method for guaranteeing a quality of service in astorage device is provided. A performance parameter for each of aplurality of read-write mechanisms of the storage device is monitoredusing a quality monitoring and book-keeping unit. A quality indicatorthat is representative of the monitored performance parameter iscalculated for each of the plurality of read-write mechanisms. Eachcalculated quality indicator is associated to its read-write mechanism.One or more of the plurality of read-write mechanisms are scheduled foruse using a scheduling unit according to the assigned quality indicator.

In another embodiment, storage device has a storage medium, one or moreread-write mechanisms, a quality monitoring and book-keeping unit and ascheduling unit. The storage medium is divided into a plurality ofpartitions. The one or more read-write mechanisms are associated withthe plurality of partitions and adapted to read and write data to andfrom the plurality of partitions. The quality monitoring andbook-keeping unit is adapted to monitor an operational quality of eachpartition of the plurality of partitions and to associate each partitionwith its monitored operational quality. The scheduling unit is adaptedto schedule use of a partition of the plurality of partitions accordingto its associated operational quality.

Other features and benefits that characterize embodiments of the presentinvention will be apparent upon reading the following detaileddescription and review of the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a disc drive on which embodiments of thepresent invention may be employed.

FIG. 2 is a simplified flow diagram of a process for determining aquality indicator for each read-write mechanism of a storage deviceaccording to an embodiment of the present invention.

FIG. 3 is a simplified flow diagram of a process for schedulingread-write mechanisms for read-write operations in the storage devicebased on a quality indicator according to an embodiment of the presentinvention.

FIG. 4 is a simplified block diagram of a multi-platter storage device,such as the storage device of FIG. 1, with read-write mechanismsscheduling according to an embodiment of the present invention.

FIG. 5 is a simplified block diagram of a probe storage device withprobe tip scheduling according to an embodiment of the presentinvention.

FIG. 6 is a simplified block diagram of a raid storage system withstorage unit scheduling according to an embodiment of the presentinvention.

FIG. 7 is a single platter storage system with platter partitioningaccording to an embodiment of the present invention.

FIG. 8 is a simplified example block diagram of a detection system.

FIG. 9 is a log graph of bit error rate (BER) versus signal-to-noiseratio (SNR) performance for different signal scales.

FIG. 10 is a graph of an average number of errors within a special markversus signal scale for signal-to-noise ratio equal to 17 dB accordingto an embodiment of the present invention.

FIG. 11 is a graph of average path metric versus signal scale for SNRequal to 17 dB according to an embodiment of the present invention.

FIG. 12 is a graph of individual path metrics distributions for no wearand tear case and for a 30% wear and tear case versus a special marknumber at a SNR equal to 17 dB.

FIG. 13 is a probability density function magnitude versus path metricrange graph of path metrics corresponding to signal scale equal to 1 andsignal scale equal to 0.7 at an SNR equal to 17 dB according to anembodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 is an isometric view of a disc drive 100 in which embodiments ofthe present invention are useful. Disc drive 100 includes a housing witha base 102 and a top cover (not shown). Disc drive 100 further includesa disc pack 106, which is mounted on a spindle motor (not shown) by adisc clamp 108. Disc pack 106 includes a plurality of individual discs107, which are mounted for co-rotation about central axis 109. Each discsurface has an associated disc head slider 110 to which a read/writetransducer (not shown) is mounted, for communication with the discsurface.

In the example shown in FIG. 1, disc head sliders 110 are supported bysuspensions 112 which are in turn attached to track accessing arms 114of an actuator 116. The actuator shown in FIG. 1 is of the type known asa rotary moving coil actuator and includes a voice coil motor (VCM),shown generally at 118. Voice coil motor 118 rotates actuator 116 withits attached disc head sliders 110 (and associated read/writetransducers) about a pivot shaft 120 to position the disc head slider110 over a desired data track along an arcuate path 122 between a discinner diameter 124 and a disc outer diameter 126. Voice coil motor 118is driven by electronics 130 based on signals generated by heads 110 anda host system 101. The host system 101 is coupled to the disc drive 100via an interface 103. Data and control signals pass from the host system101 to the disc drive 100 over the interface 103.

In general, the present invention provides systems and methods formaximizing a lifetime of a storage device by monitoring readback signalquality, tracking read-write mechanism performance, and scheduling theread-write mechanisms to ensure a desired QoS. The present invention canbe applied to any type of storage device that has a read-writemechanism, such as a probe tip, a magnetic read-write head, alaser-optical read-write mechanism, and the like. As used herein, theterm “read-write mechanism” refers to any element within a storagedevice adapted to read data from and/or write data to a storage medium.

FIG. 2 is a simplified flow diagram of a process for determining aquality indicator for each read-write mechanism of a storage device. Theread-write mechanism can be a magnetic read-write head, a probe tip, orany other mechanism adapted to read and to write data to and from astorage medium of a storage device. First, the system decodes a receivedsignal or data sequence (step 200). The decoding process may includeadjusting an adaptive gain controller (AGC) and timing recovery blocksusing a preamble portion of the received signal or data sequence. Aquality monitoring and book-keeping unit detects a special mark withinthe decoded signal or data sequence (step 202). The quality monitoringand book-keeping unit determines an operational quality of a read-writemechanism that has read the special mark (step 204). The operationalquality can be determined based on a correlation between the detectedspecial mark and the actual special mark, based on a number of errors inthe special mark, based on a path metric of the special mark, or basedon other indicators. The quality monitoring and book-keeping unit storesquality metrics for each read-write mechanism in a memory unit (step206). The quality metrics are based on the determined operationalquality of each read-write mechanism. The quality metrics can includenumbers of errors of a read-write mechanism, log-likelihood ratios,special mark detection errors, and the like. Finally, after a period ofoperation (such as after 100 read-write accesses), a quality indicatorfor each read-write mechanism is computed based on the quality metrics(step 208), and the quality indicator is stored in the memory unit (step210). The quality indicator is representative of an operational qualityor operational reliability of the read-write mechanism.

The process for extracting the operational quality/reliability of eachread-write mechanism can vary slightly according to the algorithm usedfor channel detection. In general, the quality monitoring andbook-keeping unit monitors an operational quality of each read-writemechanism. This operational quality can be a number of output errors forthe special mark, for example. In one embodiment, the correlation of thereceived signal corresponding to the special mark and the decodedspecial mark is used to extract the operational quality for theread-write mechanism, such as numbers of errors. This method has a lowcomplexity.

If a Viterbi-like sequence detection method is used to detect thespecial mark for each read-write mechanism, a minimum path metric foreach read-write mechanism can be examined at the end of the special markdetection process. An operational quality of each read-write mechanismcan be determined based on this minimum path metric. For example, theread-write mechanisms can be ranked in ascending order based on theirrespective path metrics or an average of the path metrics over a periodof time. This ranked order can then be used by a scheduler to scheduleuse of each read-write mechanism. The complexity of the Viterbiimplementation depends on the equalizer target response employed foreach channel, and can range from medium to high.

In another embodiment, the operational quality can be based on a numberof errors in the decoded special mark. The complexity of this methoddepends on what kind of detection method is used. If the detectionmethod is a simple threshold detector, the complexity will be low.However, if it is a Viterbi-like detector, the complexity might rangefrom medium to high. In another embodiment, the Log-Likelihood Ratios(LLRs) from a Soft Input Soft Output (SIS) detector, such as a SoftOutput Viterbi Algorithm (SOVA), are used to decide on the quality ofthe read-write mechanism during the detection of special mark. Thecomplexity of SISO detectors can be very high compared to conventionalViterbi detectors.

In one embodiment, the robustness of the quality/reliability detectioncan be improved by utilizing a combination of one or more of the numberof errors, minimum path metrics, LLRs, threshold, and correlation foreach read-write mechanism. For example, the quality/reliabilitydetection can utilize both the path metrics of each read-write mechanismand also a number of errors during detection of the special mark foreach read-write mechanism to extract the operational quality and todetermine the quality metric. The complexity of this method is a sum ofthe complexities of the methods used.

As a result of this monitoring process, the associated quality metrics,such as a number of errors, LLRs, and the like, are recorded in thememory unit, which tracks the operation reliability of each read-writemechanism. As previously described with respect to step 208 above, aftera certain period of operation (such as after 100 read-write accesses), aquality indicator Q(i) is calculated for each read-write mechanism. Forexample, if a number of special mark detector errors were employed asthe quality metric, the quality indicator Q(i) can be calculated to beproportional to an average number of special mark detection errors. Theaverage number of errors can be normalized over a number of read/writeaccesses (M), such as over the last 100 read-write accesses.

It should be understood that the specific order of steps in FIG. 2 canvary and that steps can be combined, depending on the implementation.For example, the extraction of the operational quality (step 204) andthe detection of the special mark (step 202) can be combined into asingle step. Additionally, the quality metrics can be temporarily storedin a buffer of the quality monitoring and book-keeping unit untilsufficient data is available to calculate the quality indicator based onthe quality metrics. Then the quality metrics and the quality indicatorcan be stored in the memory.

FIG. 3 is a simplified flow diagram of a scheduling strategy forscheduling read-write mechanisms of a storage device according to anembodiment of the present invention. The quality indicator matrix Q(i),where i=1, . . . , N can be utilized to facilitate the scheduling of theread-write mechanisms of a storage device. A read-write mechanismscheduler ranks each read-write mechanism of the storage deviceaccording to its associated quality indicator Q(i) (step 300). The ranksare then compared against a pre-determined threshold quality (step 302).If a read-write mechanism has a rank that falls below the pre-determinedthreshold quality (step 304), the read-write mechanism is declarednon-operational, and future access via that read-write mechanism isprohibited (step 306). For example, if the quality indicator (Q) of aread-write mechanism falls below a pre-defined threshold (q), the dataassociated with read-mechanism are moved to other read-write mechanismsthat have acceptable quality indicator (Q) values. In addition, readmechanism with the low quality indicator is declared non-operational andits future access is prohibited. This strategy maximizes the devicelifetime by preventing the storage device from failing due only to a fewmalfunctioning read-write mechanisms.

If a read-write mechanism has a rank that is above the predeterminedthreshold quality (step 304), the scheduler checks to see ifobject-based storage is applicable (step 308). If object-based storageis applicable (step 309), the scheduler schedules the read-writemechanisms according to the desired QoS (step 310). For example, if thecurrent object is critical to the end user, the system can demand thebest available reliability or QoS. The scheduler can then schedule theread-write mechanisms that possess the best Q values to store theobject. In another scenario, if the access data rate is more importantthan reliability for the current object, the scheduler activates allavailable read-write mechanisms with acceptable Q values to write theobject. Subsequently, when the object is accessed, the device can readback the object faster using all available read-write mechanismssimultaneously. In general, the read-write mechanism (i) is scheduled ifa cost function ƒ(x,y) is minimized based on the quality factor, suchthat ƒ(Q(i)) is minimized.

If object-based storage is not applicable (step 309), the schedulerallocates a number (k) of read-write mechanisms with the highest qualityindicator values (step 312). For example, if during normal operation, anumber (K) of read-write mechanisms (N) (where K is less than N) isrequired for writing, the system allocates the number (K) of read-writemechanisms that are available for writing with the highest Q values. Ifthe reliability degradation of each read-write mechanism is proportionalto the access frequency, the scheduler distributes the access frequencyof the read-write mechanisms over the total number (N) of availableread-write mechanisms, guaranteeing that the end user always experiencesthe highest reliability available from the device.

It should be understood that the order of the steps described withrespect to FIG. 3 above can vary, depending on the implementation. Forexample, steps 302 to 306 can be performed separately from the ordinaryread-write scheduling of the read-write mechanism, such as when thestorage device is not being accessed. Thus, when a write instruction isreceived, the failed or failing read-write mechanisms are alreadyremoved from service, thereby allowing the scheduler to scheduleread-write mechanisms for use based solely on the ranking of the activeread-write mechanisms. Additionally, scheduler can check if object-basedstorage is required before ranking the active read-write mechanisms.

FIG. 4 is simplified block diagram of a portion of a disc drive system400 with so read-write monitoring and scheduling features according toan embodiment of the present invention. The disc drive system 400includes a plurality of rotatable discs 402 arranged in a disc pack(such as disc pack 106 in FIG. 1), an actuator 404, a plurality of trackaccessing arms 406, a plurality of suspensions 408 with associatedread-write mechanisms 410 (disposed on a disc-head slider), decoders412, a quality monitoring and book-keeping unit 414, a memory unit 416,a scheduling unit 418, and a read-write control circuit 420.

Each of the plurality of read-write mechanisms 410 is associated withone surface of one disc of the plurality of rotatable discs 402. Adecoder 412 is coupled to each of the read-write mechanisms 410 todecode data read from the rotatable disc 402 into user data. The qualitymonitoring and book-keeping unit 414 monitors the decoded user datasignal for at least one performance parameter associated with theread-write mechanisms 410, such as signal strength deterioration, numberof errors, minimum path metrics, and the like. Results from the qualitymonitoring and book-keeping unit 414 are stored in the memory unit 416(which can be part of the quality monitoring and book-keeping unit 414)for tracking the operational reliability of each read-write mechanisms410. The scheduling unit 418 is adapted to use the stored results toschedule the read-write mechanisms 410. The read-write control circuitry420 then activates the particular read-write mechanisms 410 toread/write data to the associated rotatable disc 402 according to theoutput from the scheduling unit 418. The read-write control circuitry420 can also select from available read-write mechanisms 410 accordingto the output from the scheduling unit 418 and to a desired quality ofservice. Thus, a particular operation requiring a high guarantee ofservice can select the highest ranked read-write mechanisms 410 from theoutput of the scheduling unit 418.

In general, the quality monitoring and book-keeping unit 414 monitorsthe decoded signal generated by the decoder 412 for each read-writemechanism 410 during operation. Over time, the quality monitoring andbook-keeping unit 414 stores quality metrics in the memory unit 416 foreach read-write mechanism 410, and develops a quality factor for eachread-write mechanism 410. The scheduling unit 418 ranks the variousread-write mechanisms 410 according to their associated quality factor,and schedules the use of the read-write mechanisms 410 based on the QoSdesired for the data to be written and based on the ranking of theread-write mechanisms 410.

FIG. 5 is a simplified block diagram of a probe storage device 500 withprobe tip scheduling according to an embodiment of the presentinvention. The probe storage device 500 includes a storage medium 502, aplurality of probe tips 504, channel decoders 506, a quality monitoringand book-keeping unit 508, a memory unit 510, a probe tip schedulingunit 512, and a read-write control circuit 514. The probe tips 504 areadapted to read data from and to write data to the storage medium 502.Each probe tip 504 is coupled to a channel decoder 506. Each channeldecoder is adapted to decode data, which is read from the storage medium502 by a probe tip 504, into user data. The quality monitoring andbook-keeping unit 508 monitors the decoded data signal and recordsquality metrics for each of the probe tips 504 in memory unit 510. Aftera period of operation, the quality monitoring and book-keeping unit 508calculates for each probe tip 504 a quality factor, which is also storedin memory unit 510. The probe tip scheduling unit 512 ranks the probetips 504 according to their associated quality factors and schedules theuse of the probe tips 504 according to the desired policy. Finally, theread-write control circuit 514 activates the probe tips 504 to readand/or to write data according to the output of the scheduling unit 512.

As previously mentioned, the read-write mechanism of the storage device,whether it is a probe tip-based device, a magnetic read-writemechanisms-based device, or any other type of storage device, canexperience wear and tear. For probe storage devices, such as probestorage device 500, the signal and noise characteristics are closelyrelated to the size of the probe tip 504, the shape of the probe tip504, and proximity of the probe tip 504 to the media. For example, wearof a probe tip 504 can result in a significantly larger tip size.Consequently, signals from neighboring tracks on the storage medium 502can interfere with the decoding of a current track. By scheduling probetips 504 based on its associated quality factor, problems associatedwith wear and tear of the probe tips 504 can be avoided.

FIG. 6 is a simplified block diagram of a redundant array of inexpensivediscs (RAID) storage system 600 with storage unit scheduling accordingto an embodiment of the present invention. The RAID storage system 600includes a plurality of data drives 602A-602D. Bach data dive 602A-602Dis coupled to a corresponding intelligent unit 604A-604D. The RAIDstorage system 600 also includes a quality monitoring and book-keepingunit 606, a scheduling unit 608, a read-write control unit 610, and adata bus 612.

Generally, the intelligent unit 604 senses the operational reliabilityof its associated data drive 602 and identifies the state of the datadrive 602. The quality monitoring and book-keeping unit 606 monitors theindividual operational reliability of each data drive 602. Thescheduling unit 608 ranks each data drive 602 according to itsdetermined operational reliability and schedules the data drives 602according to a desired level of quality. The read-write control unit 610writes data to the data drives 602 according to the output of thescheduling unit 608. For example, the scheduling unit 608 can beconfigured to generate an output of the data drives 602 that have anoperational reliability (or quality factor) that is above apredetermined threshold. The read-write control unit 610 can then writeto the listed drives. If the quality monitoring information indicatesthat the associated drive is deteriorating, then the quality monitoringand book-keeping unit 606 can instruct the read-write control unit tomove critical information from the deteriorating drive (for example,data drive 602A) to one or more of the other more reliable data drives(such as data drives 602B-602D). Alternatively, the read-write controlcircuit 610 can be adapted to utilize different error correction coding(ECC) schemes for data written to the deteriorating data drive. In oneembodiment, the read-write control unit 610 can be adapted to moveinformation from the deteriorating drive to another drive. Finally, thescheduling unit 608 can be adapted to use a variety of schedulingschemes, depending on the specific application and the importance of thedata for a given application.

While the RAID storage system 600 is depicted as having four data drives602A-602D, it should be understood that any number of data drives 602can be included. The quality monitoring and book-keeping unit 606 isadapted to monitor each of the available data drives 602 based oninformation from their associated intelligent unit 604. Additionally,the scheduling unit 608 is adapted to schedule data drive use among anynumber of available data drives 602.

FIG. 7 is a single platter storage system 700 with platter partitioningaccording to an embodiment of the present invention. The single plattersystem 700 includes a storage medium 702, which is partitioned into sixdisjoint physical regions that correspond to four functional partitions(P1, P2, P3 and P4). The single platter storage system 700 also includesa read-write mechanism 704, a channel decoder 706, a quality monitoringand book-keeping unit 708, a partition scheduling unit 710, and aread-write control unit 712.

It should be understood by workers skilled in the art that the storagemedium 702 can be divided into any number of partitions, which can bedistributed in any arrangement on the storage medium 702. Thesepartitions (P1-P4) can be defined on the storage medium 702 of thesingle platter storage system 700 when the storage system 700 ismanufactured, for example, during a post-assembly defect scanningprocess. Additionally, quality of the partitions (P1-P4) of the storagemedium 702 can be identified during operation, and the qualitymonitoring and book-keeping unit 708 can calculate a quality factor foreach partition. The partition scheduling unit 710 can then schedule apartition (P1, P2, P3, or P4) based on a desired QoS or based on ascheduling algorithm. In this instance, each partition (P1-P4) istreated as an independent unit.

First, the read-write mechanism 704 reads data from the storage medium702. The channel decoder 706 processes the readback signal r(t)corresponding to a data sector at a particular position on the storagemedium 702. The quality monitoring and book-keeping unit 708 monitorsthe operational reliability of the particular region and position fromis which the data is read. The partition scheduling unit 710 updates theschedules of the existing partitions (P1-P4) and/or identifies a newfunctional partition according to the reliability of the particularlocation. Finally, the data is provided to the read-write mechanism 704by the read-write control unit 712 for writing to a selected partition(P1, P2, P3, or P4), according to a scheduling algorithm.

FIG. 8 is an illustrative, simplified example block diagram of adetection system 800. The detection system 800 includes a targetresponse filter 802 and a Viterbi detector 804. A data signal x(k) isconvolved to a target by the target response filter 802, resulting in aconvolved output signal y(k). The output signal y(k) and channel noisen(k) combine to form a readback signal r(k), which is processed by theViterbi detector 804 to estimate an output sequence x′(k) that isrepresentative of the data signal x(k).

Generally, SNR degradation of read-write mechanisms can be detected byidentifying an amplitude reduction in the readback signal r(k), whetherthe read-write mechanism is a probe tip, a magnetic read-writemechanism, or another read-write device adapted to read and write datato a storage medium. It is assumed that the received readback signalr(k) contains inter-symbol interference (ISI) noise. The followingdiscussion is based on a system, such as that shown in FIG. 8, where thetarget response filter 802 uses a normalized version of [1 2 1] (i.e.,0.41 0.82 0.41]) as the fixed equalizer target response. The Viterbidetector 804 applies a Viterbi algorithm corresponding to that targetresponse.

It is assumed that the noise source n(k) in the system is additive whiteGaussian noise (AWGN), which is added at the input of the detector. Thestandard deviation σ of AWGN can be found using the followingexpression:

$\sigma = \sqrt{\frac{E_{s}}{10^{{SNR}_{e}/10}}}$where E_(s) is the energy of the target response, which is unity as withthe normalized the target response, and SNR_(e) corresponds to theelectronics noise SNR in the system.

FIG. 9 is a log graph of BER versus SNR performance of an embodiment ofthe present invention for different signal scales, corresponding todifferent amounts of wear and tear associated with probe tips. The wearand tear of the read-write mechanism can be modeled using signalamplitude reduction. More specifically, the target response [0.41 0.820.41] is multiplied by a signal scale, ranging from 0 to 1. For example,if there is no wear and tear on the read-write mechanism or the storagemedium, the value of the signal scale is one. By contrast, if theread-write mechanism is not functioning at all, the signal scalereceives a value of zero. As expected, the wear and tear significantlyaffects the system performance. At an SNR of approximately 17 on alinear scale, the storage device experiences a BER of approximately9×10⁻⁵ BER for a read-write mechanism having no wear and tear (signalscale equal to one). However, each 0.1 reduction in the signal scalecosts approximately one decade in terms of the BER. Thus, at a wear andtear value of 0.9, the BER is approximately 8.5×10⁻⁴.

To determine the signal scale, the methods described with respect toFIG. 2 were applied. For example, the special mark was set to be thefollowing bit sequence

1 1 −1 −1 1 1 −1 −1 −1 −1 −1 −1 −1 1 1 1 1 1 −1 −1 −1 −1 −1 1 1,

which has a length of 27 bits. The SNR is taken to be 17 dB and thespecial marks are read 1000 times. The number of errors are tabulated.

FIG. 10 is a graph that illustrates the average number of errors withinthe special mark as a function of signal scale according to anembodiment of the present invention. When the signal scale is between0.8 and 1, the average number of errors is approximately zero. When thesignal scale is 0 or 0.1, there is an average of approximately 15 errorsin the special mark. Thus, it is easy to differentiate a case where theread-write mechanism has no wear and tear from a case where theread-write mechanism has severe wear and tear. However, on this scale,it is relatively difficult to distinguish cases where the read-writemechanism has intermediate wear and tear. However, by exploiting thepath metrics of the Viterbi detector, it is possible to differentiatethe cases much more precisely, making it possible to rank theoperational reliability of the read-write mechanisms.

FIG. 11 is a graph of average path metric at the end of the special markas a function of signal scale for an embodiment of the present inventionwith SNR equal to 17 dB. In this instance, the average path metricchanges fast as the value of the signal scale decreases from 1 to 0.6.Thus, while it is difficult to distinguish between 0.6 and 1 in terms ofthe average number of errors (as shown in FIG. 10), the average pathmetric provides a means for identifying the operational quality for thesignal scale between 0.6 and 1.

FIG. 12 is a graph of values of the individual path metrics for aread-write mechanism having different wear and tear values versus aspecial mark number for an SNR of approximately 17 dB. Generally, thedistributions of the individual path metrics for a read-write mechanismwith no wear and tear (signal scale 1) has an average path metric thatis between zero and one. The mean and standard deviations of the pathmetric values for the no wear and tear case is 0.5373 and 0.1490,respectively. By contrast, a read-write mechanism with 30 percent wearand tear (signal scale=0.7) has a mean of about 3.5127 and a path metricdeviation of about 0.4094. The separation between the distributionsprovides a good indicator of wear.

FIG. 13 is a graph of probability density functions (PDFs) for thevalues shown in FIG. 12, assuming a Gaussian PDF distribution. The graphis a probability density function magnitude versus path metric rangecorresponding to signal scale equal to 1 and signal scale equal to 0.7at an SNR equal to 17 dB. The task of distinguishing between a signalscale of 1 and a signal scale of 0.7 is a classical detection problem.For example, it is possible to compare the average path metric for aread-write mechanism to a threshold of 1.35 (where the two probabilitydensity functions have approximately the same value). If the path metricis larger than 1.35, it can be detected as having more than 30 percentwear and tear. Otherwise, it could be assumed that there is no obviouswear and tear, particularly if a desired QoS would permit wear and tearof up to about 30 percent.

Moreover, the probability of misjudgment can be estimatedstraightforwardly. For example, given that the read-write mechanism hasapproximately 30 percent wear and tear, the probability that read-writemechanism is misclassified as having no wear and tear is approximately6.52×10⁻⁸. On the other hand, given the read-write mechanism has no wearand tear, the probability that it will be misclassified as having 30percent wear and tear is approximately 2.30×10⁻⁸. These misjudgmentvalues can become significantly lower if average path metric values areused to compare with the threshold (after a certain number of accesstimes). In general, the threshold value and the associated misjudgmentprobabilities highly correlate with the system characteristics, such astypical SNR values, noise mixtures, and the like. Depending on thedesired QoS, the system can be initialized such that the values are setaccording to the systems typical behavior and the desired schedulingpolicy.

For example, if the quality factor Q(i) assumes discrete values 1, 0.7,and 0, the quality factor Q(i) can correspond to signal scale factor (C)having values within one of three ranges: 0.7<C≦1; 0.5<C≦0.7; and0≦C≦0.5, respectively. A quality factor Q(i) can then be assigned toeach read-write mechanism (i) as follows. If the average number ofspecial mark errors for a particular read-write mechanism is greaterthan 9, the quality factor Q(i) for the read-write mechanism is assignedas zero. If the average number of special mark errors is less than 9 andthe average path metric in the Viterbi detector for special mark islarger than 1.35, the quality factor Q(i) for the read-write mechanismis assigned as 0.7. If the average number of special mark errors is lessthan 9 and the average path metric is less than 1.35, the quality factorQ(i) for the read-write mechanism is assigned as 1. Subsequently, thequality factor Q(i) values can be exploited according to the desiredread-write mechanism scheduling policy. In one embodiment, thescheduling unit can be adapted to move data associated with read-writemechanisms that have a lower Q value to other read-write mechanisms withhigher Q values, thereby improving the overall QoS of the system. Inanother application, usage of read-write mechanisms with Q values belowa predetermined threshold value (for example, below a threshold valueof 1) can be prohibited using the scheduling unit. Such a schedulingpolicy avoids utilizing read-write mechanisms that have degraded,thereby helping to prevent permanent data loss.

While the present invention has been described with respect to specificstorage device implementations, it should be understood by workersskilled in the art that the quality monitoring and book-keeping featuresof the present invention can be implemented on any channel detectionsystem or any storage device. Additionally, though present invention hasbeen described with respect to Viterbi-type channel decoders, thesystems and methods of the present invention can be implemented with anytype of channel decoding scheme. Finally, though various techniques foridentifying a quality factor for each read-write mechanism are describedabove, it should be understood that any algorithm for calculating aquality factor can be utilized. In a preferred embodiment, the qualityfactor determination allows the system to distinguish between levels ofdeterioration.

It is to be understood that even though numerous characteristics andadvantages of various embodiments of the invention have been set forthin the foregoing description, together with details of the structure andfunction of various embodiments of the invention, this disclosure isillustrative only, and changes may be made in detail, especially inmatters of structure and arrangement of parts within the principles ofthe present invention to the full extent indicated by the broad generalmeaning of the terms in which the appended claims are expressed. Forexample, the particular elements may vary depending on the particularapplication for the QoS-guaranteed storage device system whilemaintaining substantially the same functionality without departing fromthe scope and spirit of the present invention. In addition, although thepreferred embodiment described herein is directed to a storage devicesystem for guaranteeing a desired QoS by monitoring an operationalquality for each read-write mechanism and by scheduling read-writemechanisms for use based on the operational quality, it will beappreciated by those skilled in the art that the teachings of thepresent invention can be applied to any type of channel detection systemthat can experience physical deterioration over time, without departingfrom the scope and spirit of the present invention.

1. A storage device comprising: a storage medium divided into aplurality of partitions; one or more read-write mechanisms associatedwith the plurality of partitions and adapted to read and write data toand from the plurality of partitions; and a quality monitoring andbook-keeping unit adapted to monitor an operational quality of eachpartition of the plurality of partitions and to associate each partitionwith its monitored operational quality; and a scheduling unit adapted toschedule use of a partition of the plurality of partitions according toits associated operational quality.
 2. The storage device of claim 1wherein the quality monitoring and book-keeping unit further comprises:a memory unit for storing the monitored operational quality for eachpartition.
 3. The storage device of claim 1 wherein the scheduling unitis adapted to rank the plurality of partitions according to themonitored operational quality and to generate a partition scheduleoutput based on the ranking.
 4. The storage device of claim 3 furthercomprising: read-write control circuitry coupled to the scheduling unitand adapted to use the partition schedule output to select a partitionof the plurality of partitions for use based on a desired quality ofservice.
 5. The storage device of claim 1 wherein the quality monitoringand book-keeping unit is adapted to calculate a quality indicator foreach partition based on the monitored operational quality.
 6. Thestorage device of claim 5 wherein the quality indicator for eachpartition is computed by the quality monitoring and book-keeping unitbased on an average number of detected errors from received signals overa period of time.
 7. The storage device of claim 1 wherein the schedulerdeclares a selected partition of the plurality of partitions to benon-operational and prohibits future use of the selected partition ifthe monitored operational quality falls below a pre-determinedthreshold.